Plasmaspheric hiss1 is a type of electromagnetic wave found ubiquitously in the dense plasma region that encircles the Earth, known as the plasmasphere2. This important wave is known to remove3, 4, 5 the high-energy electrons that are trapped along the Earth's magnetic field lines6, and therefore helps to reduce the radiation hazards to satellites and humans in space. Numerous theories to explain the origin of hiss have been proposed over the past four decades, but none have been able to account fully for its observed properties. Here we show that a different wave type called chorus7, 8, previously thought to be unrelated to hiss, can propagate into the plasmasphere from tens of thousands of kilometres away, and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day–night asymmetry in intensity, its association with solar activity and its spatial distribution. The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere9, whereas hiss depletes these electrons at lower equatorial altitudes3, 4
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